Myxoid Neurofibroma: Histopathology, IHC, and Differential Diagnosis

Overview

Histopathological Findings

Architecture (Low Power)

• Non-encapsulated, unencapsulated dermal lesion, typically well-demarcated at low magnification
• Centered in the dermis but may extend into superficial subcutis
• The stroma is predominantly myxoid/mucinous, with the myxoid matrix being the hallmark feature - described as "extensive deposition of stromal mucin"
• Low to moderate overall cellularity (often strikingly hypocellular, which creates diagnostic difficulty on FNA/cytology)
• No true capsule; may have a peripheral condensation of fibrous tissue

Cellular Features (High Power)

• Cells have wavy, serpentine, or "comma-shaped" nuclei - a defining feature of Schwann cell lineage
• Scant, poorly-defined cytoplasm; cell borders are indistinct
• Cells are haphazardly arranged in the mucinous matrix, not forming tight fascicles
• Single-layer thin fascicles of spindle cells are present
• Nuclear pleomorphism may occasionally be present (degenerative atypia), but significant atypia raises the concern for MPNST
• Mitoses are absent or extremely rare (< 1 per 50 HPF is the accepted threshold)
• Mast cells are scattered throughout the stroma - a useful diagnostic clue favoring neurofibroma over its mimics
• Vacuolated histiocytes ("muciphages" or "pseudolipoblasts") may be present within the mucinous pools

Stromal Features

• Matrix is rich in hyaluronic acid-type mucin (stains with Alcian blue, colloidal iron)
• Ropey collagen bundles ("shredded collagen") are intermingled within the mucoid background - a characteristic finding highlighted in cytology specimens
• Thin-walled blood vessels and scattered lymphocytes may be present
• In diffuse-type myxoid neurofibroma, a branching plexiform vascular network may be prominent, mimicking myxoid liposarcoma

Axons

• Axons course through the tumor mass - a unique feature compared to most mimics
• These are difficult to see on H&E but are demonstrable with neurofilament immunostain
• The ratio of axons to Schwann cells is < 1:1 (distinguishing neurofibroma from neuroma)

Additional Notes

• Floret-like multinucleated giant cells (CD34+) may be present, as in spindle cell/pleomorphic lipoma
• Organoid pseudo-Meissnerian body-like structures may be seen, especially in diffuse type

Source: Fitzpatrick's Dermatology, Histopathology section; MDPI Diagnostics 2022 (Practical Approach to Peripheral Nerve Sheath Tumors)

Immunohistochemistry (IHC) Profile

• The combination of S100 (+) + SOX10 (+) + CD34 (variable) + EMA (focal perineurial) is characteristic
• The Schwann cell population is the dominant S100/SOX10-positive component
• CD34 positivity in scattered fibroblast-like cells is helpful in supporting nerve sheath lineage
• MART-1/Melan-A, p53, cytokeratin, and desmin are negative - helping exclude melanocytic, epithelial, and smooth muscle lesions
• The pattern of S100(+) and SOX10(+) with absent or only focal EMA helps separate from perineurioma (EMA+/S100-)

Source: Fitzpatrick's Dermatology, IHC Table; MDPI Diagnostics 2022; [Shaker et al., Diagnostic Cytopathology 2024, PMID 38363063]

Differential Diagnosis

1. Nerve Sheath Myxoma (True Neurothekeoma / NSM)

• Historically conflated with myxoid neurofibroma; now recognized as a distinct entity
• Architecture: multilobulated/multinodular with distinct fibrous septa separating myxoid lobules - more organized than myxoid NF
• Cells: epithelioid and stellate Schwann cells in syncytial-like aggregates, ring-shaped multipolar cells
• IHC: S100 strongly positive, GFAP positive, CD57 positive; EMA-positive perineurial cells at periphery of lobules
• Distinguishing feature: the lobular architecture is the most reliable separator from myxoid NF

2. Dermatofibrosarcoma Protuberans (DFSP), Myxoid Variant

• Architecture: storiform/cartwheel pattern of spindle cells infiltrating subcutis in a honeycomb pattern
• IHC: CD34 strongly and diffusely positive; S100 negative
• Genetics: COL1A1-PDGFB translocation t(17;22) - confirmatory by FISH or RT-PCR
• Key distinction: S100 negativity and strong diffuse CD34 exclude myxoid NF

3. Superficial Angiomyxoma (Cutaneous Myxoma)

• Loosely cellular myxoid lesion with prominent thin-walled blood vessels
• Cells are stellate to spindle without wavy nuclei; no Schwann cell differentiation
• IHC: S100 negative, CD34 variable - key negative S100 separates from myxoid NF
• Associated with Carney complex (NAME/LAMB syndromes) - note these syndromes also include myxoid NF
• Mucin: hyaluronic acid-rich (same as NF); erosion/ulceration of overlying epidermis is a clue

4. Myxoid Liposarcoma

• Contains a plexiform capillary network ("chicken wire" vasculature) - the most helpful architectural feature
• Cells: primitive round to oval lipoblasts in varying stages of differentiation; univacuolated lipoblasts are characteristic
• IHC: S100 variable, MDM2/CDK4 usually negative (unlike dedifferentiated variant); DDIT3 (CHOP) nuclear expression is highly sensitive/specific ([PMID 32815829])
• Genetics: FUS-DDIT3 or EWSR1-DDIT3 translocation - definitive
• Key distinction: presence of lipoblasts + chicken-wire vasculature + DDIT3 rearrangement

5. Low-Grade Myxofibrosarcoma (Myxoid MFH)

• Curvilinear blood vessels ("pseudovascular spaces"), mild nuclear pleomorphism even at low grade
• IHC: S100 negative, CD34 negative
• Usually larger, deeper, and in older patients; higher mitotic rate than NF
• No axons

6. Low-Grade Fibromyxoid Sarcoma (Evans Tumor)

• Alternating fibrous and myxoid zones ("swirling pattern")
• Cells: bland spindle cells with little atypia; can be confused with NF in myxoid zones
• IHC: MUC4 strongly positive (highly sensitive/specific marker); S100 negative
• Genetics: FUS-CREB3L2 translocation

7. Schwannoma (Antoni B Areas)

• Antoni B areas in schwannoma are hypocellular and myxoid, closely resembling myxoid NF
• Distinguished by: encapsulation, alternating Antoni A (cellular, Verocay bodies) and Antoni B areas; the absence of axons within the lesion (unlike NF)
• IHC: S100 strongly and diffusely positive (in NF, only Schwann cells are S100+, not all cells); NF2/merlin often lost in schwannoma
• CD34: often negative in schwannoma vs. variable in NF

8. Perineurioma (Intraneural or Soft Tissue)

• Composed of perineurial cells with elongated bipolar processes
• IHC: EMA strongly positive; S100 negative; GLUT-1 positive; claudin-1 positive
• This is the clearest IHC separator: EMA+/S100- is perineurioma; S100+/EMA-or-focal is NF

9. Myxoid Neurofibroma vs. MPNST with Myxoid Change

• MPNST may arise from NF1-associated neurofibroma (especially plexiform type)
• Features of malignancy: high cellularity, marked nuclear atypia, mitoses > 4/10 HPF, necrosis, loss of H3K27me3 (trimethylation of H3 at lysine 27), loss of p16
• IHC: H3K27me3 loss is highly specific for MPNST (retained in benign NF); S100 becomes patchy/focal

Summary Table

Clinical Context

• Presents as a solitary, asymptomatic, flesh-colored to pink-blue dermal nodule, most commonly on the face, shoulders, and arms
• Higher incidence in teenagers and young adults
• May occur in periungual/subungual locations
• Association with NF1 (may or may not be present)
• Component of NAME syndrome (Nevi, Atrial myxoma, Myxoid neurofibroma, Ephelides)
• Benign clinical course; malignant transformation has not been reported for the myxoid variant
• Treatment: simple excision

• Fitzpatrick's Dermatology, 9th ed., Chapter 121 (Nerve Sheath Tumors) - histopathology and IHC tables
• Dermatology 2-Volume Set, 5th ed. (Bolognia), Chapter 115 - differential diagnosis of nerve sheath myxoma vs. neurofibroma
• [Shaker N et al., "Myxoid neurofibroma masquerading as lymphatic-venous malformation," Diagnostic Cytopathology 2024, PMID 38363063] - cytological features and IHC on excisional biopsy
• Practical Approach to Histological Diagnosis of Peripheral Nerve Sheath Tumors, Diagnostics 2022 - S100/SOX10/CD34/EMA IHC profile; diffuse type with myxoid stroma
• Histopathological Variants of Cutaneous Neurofibroma, Dermatopathology 2023 - myxoid cNF definition, NAME syndrome association, pseudolipoblasts